The present invention relates to a piston type variable displacement fluid machine. More specifically, the present invention pertains to a piston type variable displacement compressor that is used in a vehicular air-conditioner and includes piston rings each sealing the space between one of pistons and the inner circumferential surface of a corresponding cylinder bore.
A typical compressor used in a vehicular air-conditioner includes a clutch mechanism, such as an electromagnetic clutch, on a power transmission path between an external drive source, which is an engine, and the compressor. When refrigeration is not needed, the electromagnetic clutch is turned off to prevent power transmission from the engine to the compressor, thereby deactivating the compressor.
Turning on and off the electromagnetic clutch generates a shock, which lowers the driving performance of a vehicle. Therefore, clutchless type compressors are now widely being used. In a clutchless type compressor, the clutch mechanism is not arranged on the power transmission path between the engine and the compressor.
The clutchless type compressor employs a piston type variable displacement compressor that can vary the displacement by adjusting the stroke of the piston. When refrigeration is not needed, the stroke of the piston is minimized to minimize the displacement of the compressor. This minimizes the power loss of the engine.
The clutchless type compressor is always driven when the engine is running. Therefore, when the minimum displacement of the compressor is set to zero, refrigerant gas containing lubricant does not flow through the refrigeration circuit. Thus, sliding parts inside the compressor are not sufficiently lubricated.
Therefore, the minimum displacement of the compressor, or the minimum stroke of the piston, cannot be set to zero. Thus, the pistons reciprocate even when the compressor is driven at the minimum displacement. This increases the power loss of the engine by the sliding resistance caused between each piston ring and the inner circumferential surface of a corresponding cylinder bore.
In a case when carbon dioxide is used as refrigerant, the refrigerant pressure in the compression chamber is much higher than when chlorofluorocarbon is used. Therefore, to suppress blowby gas, each piston ring needs to be pressed against the inner circumferential surface of the corresponding cylinder bore with much more strength than when chlorofluorocarbon is used. This increases the power loss of the engine.
Accordingly, it is an objective of the present invention to provide a piston type variable displacement fluid machine that has reduced sliding resistance between each piston and a corresponding cylinder bore during the minimum displacement of the fluid machine.
To achieve the above objective, the present invention provides a piston type variable displacement fluid machine. The fluid machine includes a housing, a drive shaft, a cylinder bore, a piston and a piston ring. The drive shaft is rotatably supported by the housing. The cylinder bore is formed in the housing. The piston is accommodated in the cylinder bore. The cylinder bore has an inner circumferential surface and the piston has an outer circumferential surface. The piston reciprocates along a line of movement in the cylinder bore in accordance with the rotation of the drive shaft. The stroke of the piston is varied between a predetermined maximum stroke and a predetermined minimum stroke, which is greater than zero. The displacement of the fluid machine is changed in accordance with the stroke of the piston. A ring groove is formed on the outer circumferential surface of the piston. The piston ring is fitted in the ring groove. The piston ring moves with respect to the piston in the line of movement of the piston. An allowable movement amount of the piston ring with respect to the piston is greater than the minimum stroke of the piston.
The present invention also provides a piston for a piston type variable displacement fluid machine. The fluid machine includes a cylinder bore, which accommodates the piston. The cylinder bore has an inner circumferential surface. The piston has an outer circumferential surface and reciprocates along a line of movement in the cylinder bore in accordance with the rotation of a drive shaft. The stroke of the piston is varied between a predetermined maximum stroke and a predetermined minimum stroke, which is greater than zero. The displacement of the fluid machine is changed in accordance with the stroke of the piston. The piston includes a ring groove and a piston ring. The ring groove is formed on the outer circumferential surface of the piston. The ring groove faces the inner circumferential surface of the cylinder bore. The piston ring is fitted in the ring groove. The piston ring moves with respect to the piston in the line of movement of the piston. An allowable movement amount of the piston ring with respect to the piston is greater than or equal to the predetermined minimum stroke of the piston.
Other aspects and advantages of the invention will become apparent from the following description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention.